1. Field of the Invention
The present invention relates to the production of styrene polymers, copolymers, and graft copolymers. More particularly, the present invention relates to a two stage process for the production of crystal polystyrene, impact polystyrene, styrene-acrylonitrile copolymers or acrylonitrile-butadiene-styrene terpolymers.
2. Description of the Prior Art
Styrene was one of the first monomers to be polymerized. There are many polymerization processes described in the literature. Some of the earliest processes involved placing styrene monomer in a drum in the sun and allowing the monomer to polymerize, removing the solid block from the drum, and crushing the block with hammers. Another labor intensive process was the polymerization of styrene in a type of filter press which had to be disassembled after the polymerization was completed.
An early process for the preparation of crystal and impact polystyrene involved a prepolymerization step using 10% of a solvent such as ethylbenzene, followed by bulk polymerization and devolatilization to remove the ethylbenzene. Without the ethylbenzene, the viscosity of the polymer was very high and it could not be moved through the reactor-heat exchanger. The added solvent increased the cost of the process due to the solvent cost and additional solvent removal step.
Other processes for the polymerization of crystal styrene included a suspension process wherein the styrene monomer was suspended in water using tricalcium phosphate into discrete small droplets which polymerized in the presence of organic peroxides to small hard beads. This process had the disadvantages of long polymerization cycles and batch instability, which on occasion caused the formation of lollipops (i.e., agglomeration of all the beads into a large blob, usually around the agitator).
Another process for producing crystal polystyrene, called the tower process, required large reactors and fairly long residence.
Acrylonitrile-butadiene-styrene (ABS) graft copolymers were normally produced by a latex process wherein the styrene and acrylonitrile monomers were grafted onto a polybutadiene rubber latex, the latex was then coagulated, and the polymer was lubricated and stabilized before being extruded into pellets. The process was expensive because of the numerous steps, the handling of the latex, and the long cycles involved.
Thus, the above prior art processes involved long cycles and/or the use of expensive and unstable suspensions and emulsions.
In the past, attempts have been made to polymerize styrene in a twin screw extruder. These attempts have failed due to the inability to feed a liquid hydrocarbon without leakage, to totally remove the heat of polymerization in the extruder, and to remove monomer and oligomer from the polymer. These problems have prevented the commercialization of a process to polymerize styrene in a twin screw extruder.
Accordingly, it is an object of the present invention to provide a novel and improved process for the polymerization of styrene including the copolymerization of stryene and other monomer(s). Another object of the invention is to provide a rapid and inexpensive process for the polymerization or copolymerization of styrene. A further object of the invention is to provide an efficient and workable process for the polymerization or copolymerization of styrene utilizing an extruder as a reactor for completion of the polymerization.
These and other objects, as well as the scope, nature and utilization of the invention will become apparent to those skilled in the art from the following description, the drawings and the appended claims.